<html><body><style>.div1 .div2{
  background-color:#0f0
}
.div3,.div4{
  background-color:#0f0
}
div.div5{
    color:#0ff;
}
.div5 .div6{
  background-color:#0f0
}</style>
<div class="div1">hello11111</div>
<div class="div2">world22222222</div><br/>

<div class="div3">hello33333333</div>
<div class="div4">world444444444</div><br/>

<div class="div5">hello555555555
    <div class="div6">world66666666</div>
</div>
</body></html>